The present invention relates to a rolling bearing device such as rolling bearing, ball screw and linear guide. More particularly, the present invention relates to a rolling bearing device suitable for precision machine, food-processing machine, facilities for producing semiconductor element or liquid crystal panel, medical examination equipment, medical equipment having a rotary portion such as dental hand piece, fishing equipment, etc.
The present invention further relates to a rolling bearing device which is used at a high temperature and a high rotary speed such as those for aircraft jet engine, gas turbine and transmission.
A rolling bearing device for precision machine, food-processing machine, facilities for producing semiconductor element or liquid crystal panel, medical examination equipment, fishing equipment, etc. is a device subject to use in a rusting atmosphere or a device which is not allowed to undergo even slight rusting. Therefore, the constituents of such a rolling bearing have heretofore been formed by SUS440C, which is a martensite-based steel, 0.7C-13 Cr stainless steel, etc. These stainless steels are superior to SUJ2 and SCRR420, which are used as constituents of ordinary rolling bearing devices, in respect to corrosion resistance. When subjected to hardening, these stainless steels can be provided with a surface hardness of 58 or more, which is necessary for constituents of rolling bearing device, as calculated in terms of HRC (Rockwell C hardness).
However, with the recent further diversification of working atmosphere of rolling bearing device, there has been a growing demand for corrosion resistance, rolling fatigue life and reliability. Thus, the foregoing rolling bearing device comprising SUS440C or 0.7C-13Cr stainless steel cannot occasionally meet sufficiently these requirements.
For example, data equipment have been recently miniaturized more and more and thus often used in portable form that adds to possibility of being impacted when dropped or exposed to vibration. When given minute vibration or oscillation, the rolling bearing in the equipment undergoes fretting at the contact area of rolling element with bearing ring (repeated minute relative slippage of two contacting surfaces that causes abrasion), causing deterioration of acoustic properties.
A first object of the present invention is to provide a rolling bearing device excellent in corrosion resistance and fretting resistance, paying attention to the foregoing problem.
On the other hand, a rolling bearing for aircraft jet engine or gas turbine operates at a high temperature and a high rotary speed. Therefore, such a rolling bearing is required to have a sufficient resistance to hoop stress on the core thereof or have a sufficient surface hardness at high temperatures in addition to rolling fatigue life and abrasion resistance. Accordingly, as a rolling bearing material for such a purpose there has heretofore been used a high-speed steel such as AISI-M50 and M50NiL, which are excellent in heat resistance. M50NiL, if used, is subjected to surface hardening such as carburizing.
However, these high-speed steel materials are inferior to stainless steel in respect to corrosion resistance. In particular, a rolling bearing for aircraft jet engine can be easily wet with dew and thus rusted because most airports are located close to sea or it is used in an atmosphere having a big change of temperature and humidity. Further, since importance is attached to safety in the art of aircraft, the occurrence of slight rust can be judged as termination of bearing life.
Accordingly, a rolling bearing for aircraft jet engine is required to have an improved corrosion resistance. However, the foregoing stainless steel such as SUS440C not only cannot provide a sufficient resistance to hoop stress or a sufficient hardness at high temperatures but also leaves something to be desired in corrosion resistance.
On the other hand, JP-A-7-278762 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses that when a stainless steel having a predetermined composition and having a carbon content as low as 0.03% by weight or less (preferably 0.02% by weight or less) is subjected to surface hardening by nitriding instead of carburizing, the corrosion resistance of the surface portion of the stainless steel can be improved while maintaining a sufficient core roughness and surface hardness. However, nitriding can difficultly make much nitrogen to penetrate into the surface portion of the stainless steel, making it impossible to obtain a surface hardness equal to or higher than that obtained with M50 by carburizing. Accordingly, the method disclosed in this patent leaves something to be desired in life properties during use at a high temperature and a high rotary speed.
A second object of the present invention is to provide a rolling bearing device having a prolonged life during use at a high temperature and a high rotary speed and an excellent corrosion resistance, paying attention to the foregoing problem.
Further, the recent trend is for more bearings for dental hand piece to be operated at a ultrahigh rotary speed as 400,000 to 500,00 rpm. It has been thus apprehended that the resulting early damage or seizing on the rolling surface can cause the reduction of bearing life. In order to prevent the infection of HIV virus, it has recently been practice to sterilize a dental hand piece with vapor at a temperature of about 135xc2x0 C. for several minutes every use. Thus, a dental hand piece has been required to have higher corrosion resistance. Accordingly, a bearing or dental hand piece formed by a martensite-based stainless feel can leave something to be desired in corrosion resistance.
A still further object of the invention is to provide a rolling bearing device having excellent corrosion resistance and seizing resistance suitable for such a dental hand piece.
The above-mentioned object can be achieved by a rolling bearing device, according to a first aspect of the present invention, comprising:
a fixed body having a race;
a moving body having a race; and
a rolling elements rotatably interposed between the races of the fixed body and the moving body so as to make the moving body movable relative to the fixed body,
wherein the at least one of the fixed body and the moving body is obtained by working a material made of iron alloy steel having a chromium (Cr) content of from equal or more than 5% by weight to equal or less 22% by weight into a predetermined shape, and then subjecting the material to heat treatment followed by finishing, and then the race of at least one of the fixed body and the moving body thus finished has a surface hardness HRC 58 or more, and
wherein an oxide film is formed on the race of the at least one of the fixed body and moving body and also comprises chromium atom and iron atom present therein at a ratio (Cr/Fe) of 0.5 or more.
In addition to this, the above-mentioned object can also be achieved by a rolling bearing device, according to a second aspect of the present invention, comprising:
a fixed body having a race;
a moving body having a race;
a rolling elements rotatably interposed between the races of the fixed body and the moving body so as to make the moving body movable relative to the fixed body,
wherein at least one of the fixed body and the moving body is obtained by
working a material into a predetermined shape, the material being made of alloy steel containing, 0.2% or less by weight of C; 0.1% to 2.0% by weight of Si; 10.0% to 20.0% by weight of Cr; 0.4% to 3.0% by weight of Mo; 1.0% to 3.5% by weight of Ni; 1.0% to 10.0% by weight of Co; and 0.4% to 2.0% by weight of V, and being subjected to heat treatment including carburizing or carbonitriding followed by finishing,
wherein the at least one of the fixed body and the moving body has a surface portion at its race, the surface portion having the carbon content from 1.0% to 3.0% by weight at its race,
the surface hardness of the surface portion is equal or more than HRC 62, and
an oxide film is formed on the at least one of the fixed body and moving body and also comprises chromium atom and iron atom present therein at a ratio (Cr/Fe) of not lower than 0.5, the oxide film having a thickness of not lower than 10 xc3x85.